To save power and increase data handling capacity per channel, optical data transmission systems have multiple single-ended receiver channels. Typically, the optical receiver channel has a wide-bandwidth linear single-to-differential radio frequency (RF) variable gain amplifier at its Analog Front-End (AFE). The AFE converts photodiode (e.g., PIN current) to voltage. There are two things of interest with respect to traditional AFEs: (1) high bandwidth and pass-band flatness and (2) linearity.
Regarding the high bandwidth and pass-band flatness, unlike conventional Low Noise Amplifiers (LNA), which are terminated with a 50 ohm transmission line at the input, the TIA has a large photodiode capacitance at its input. Hence, the input PIN diode capacitance sets the TIA bandwidth and contributes to the dominant-pole of the amplifier. Modern optical communication systems transmit data at 400 Gbps or higher and have multiple receiver channels receiving 28 Gbps per channel. This means that the TIA used in the optical receiver channel requires more than 20 GHz to 25 GHz of bandwidth. Large photodiode capacitance reduces the TIA bandwidth and, therefore, reduces the optical receiver data rate. Also, any peaking in the frequency response will lead to overshoots in the transient waveform leading to eye closure.
Non-linearity is also of importance because it leads to in-band distortion components. Moreover, modern optical data transmission system using complex modulation schemes (e.g., PAM-4) require lower non-linearity (DNL), which is the performance metric used to quantify the distortion in a Data Eye-Diagram. Higher DNL creates a distorted Data Eye, which results in bit errors that are not acceptable in secured communication links.